Prefrontal activity during serial probe reproduction task: encoding, mnemonic, and retrieval processes.

To study the prefrontal neuronal mechanism for the encoding and mnemonic processing of multiple objects, the order of object presentation, and the retrieval of an object among objects in the working memory, we recorded neuronal activity from the lateral prefrontal cortex while two monkeys performed the serial probe reproduction task. In the task, two objects (C1 and C2) were presented sequentially interleaved with a delay (D1) period, and after the second delay (D2) period, a color cue was presented. Monkeys were trained to select one target object on the basis of the color stimulus. During the C1 and C2 periods, we found responses that depended on the order of presentation (order-selective response). During the D1 and/or D2 periods, two-thirds of the neurons with object-selective delay-period activity showed order-selective activity coding either C1 or C2. Neurons with larger response magnitudes during the C2 period showed order-selective delay-period activity during the D2 period. These order-selective responses during the C2 period could also contribute to order-selective delay-period activity, and order-selective delay-period activity during the D1 and D2 periods could play an essential role in storing information on both the object and the temporal order of presentation. During the color cue period, two-thirds of the neurons with responses showed target object selectivity (CT and T responses), although the target object was not presented during this period. The CT and T responses could play a critical role in the retrieval of an item among various items in the working memory.